Cross-linking of high affinity IgE on mast cells results in one of the most life-threatening allergic reactions, anaphylaxis. Yet the cellular mechanisms that induce B cells to produce IgE to allergens remain poorly understood. T follicular helper (Tfh) cells are the primary T cell subset responsible for directing the affinity, longevity and isotype of antibody produced by B cells. Subsets of Tfh cells exist in both mouse and man, defined by their differential cytokine and chemokine receptor expression. Tfh2 cells are primarily identified by IL-4 expression during a type 2 immune response and can promote IgE. Whether Tfh2 cells are required for both low and high affinity IgE is not know. By studying a murine model of a rare monogenic form of IgE- mediated allergy, DOCK8 (dedicator of cytokinesis 8) deficiency, we recently discovered a new subset of Tfh2 cells that promotes high affinity IgE in vivo. We call this new subset ?TfhE? cells and identify them by the expression of a constellation of cytokine and transcription factors not normally produced by Tfh cells. Further, the same subset is induced in wild type mice, but only during aeroallergen-driven responses when high affinity IgE is produced. In contrast, TfhE cells are not induced during low affinity IgE responses. We do not know how the development of TfhE cells is regulated or how they function to induce IgE. Although IL-4 is necessary for IgE class switching, it is not sufficient in vivo. Further, the dissociation between low and high affinity production during different type 2 immune responses suggests that direct versus sequential B cell switching to IgE might be regulated by different Tfh2 subsets. The goal of our proposed experiments is to fill these crucial gaps in knowledge and thereby illuminate the pathways that drive production of high affinity IgE and the attendant anaphylactic allergic reactions. The broad impact of these advances could be substantial, enabling definition of the genetic and environmental factors that dictate high affinity IgE responses in those with asthma and other allergic conditions. We have developed tools including Tfh-restricted knockout mice to determine 1) whether DOCK8 regulates Tfh differentiation in a cell-intrinsic manner and functions similarly in human Tfh cells; 2) whether high and low affinity IgE are driven by different Tfh cell subsets in wild type mice; 3) molecular mechanisms controlling wild type TfhE development and function, including the identity of factors that, together with IL4, are produced by TfhE cells to drive IgE class switching and transcription factor(s) that dictate TfhE differentiation. If successful, these experiments will define how TfhE cells specifically promote high affinity IgE and will define molecular pathways that regulate TfhE induction. Inhibition of key pathways operational in these TfhE cells could be used to promote allergen-specific antibody isotypes that potentially block anaphylaxis. 1